The present invention relates to a vehicle powershift transmission and engine control system.
Powershift transmissions change speeds by disengaging one or more clutches and then engaging one or more clutches, with no interruption of power during the shift. Shift smoothness is obtained by modulating the pressure of the oncoming clutches, i.e. engaging them at low pressure. Shift smoothness is very sensitive to having the optimum engagement pressure in relation to the load being transmitted by the transmission. If the engagement pressure is too low, the vehicle may drop load or speed during the shift, if the engagement pressure is too high, the shift will be aggressive, jumpy and harsh. Under lightly loaded conditions, the typical engine governor response contributes to the poor shift. For example, during an upshift, if the engaging clutch locks up quickly, it will bring the engine speed down rapidly. The engine governor typically over-reacts to this drop in speed by quickly increasing fuel to the engine. An excess amount of fuel may then cause the engine to quickly overshoot the commanded speed. The resulting acceleration-deceleration causes the vehicle to pitch fore and aft, and produces a "jumpy" feel.